Method for realizing microchannels in an integrated structure

ABSTRACT

A process is presented for realizing buried microchannels in an integrated structure comprising a monocrystalline silicon substrate. The process forms in the substrate at least one trench. A microchannel is obtained starting from a small surface port of the trench by anisotropic etching of the trench. The microchannel is then completely buried in the substrate by growing a microcrystalline structure to enclose the small surface port.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a Divisional Application of U.S. applicationfor patent Ser. No. 10/726,264, filed Dec. 2, 2003, which claimspriority from European Application for Patent No. 02425746.1 filed Dec.4, 2002, the disclosures of which each being hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to a process for realizing microchannelsin an integrated structure. More specifically, the invention relates toa process for realizing microchannels buried in an integrated structurecomprising a monocrystalline silicon substrate.

2. Description of Related Art

Typical procedures for analyzing biological materials, such as nucleicacid, involve a variety of operations starting from raw material. Theseoperations may include various degrees of cell purification, lysis,amplification or purification, and analysis of the resultingamplification or purification product.

As an example, in DNA-based blood tests the samples are often purifiedby filtration, centrifugation or by electrophoresis so as to eliminateall the non-nucleated cells. Then, the remaining white blood cells arelysed using chemical, thermal or biochemical means in order to liberatethe DNA to be analyzed. Next, the DNA is denatured by thermal,biochemical or chemical processes and amplified by an amplificationreaction, such as PCR (polymerase chain reaction), LCR (ligase chainreaction), SDA (strand displacement amplification), TMA(transcription-mediated amplification), RCA (rolling circleamplification), and the like. The amplification step allows the operatorto avoid purification of the DNA being studied because the amplifiedproduct greatly exceeds the starting DNA in the sample.

The procedures are similar if RNA is to be analyzed, but more emphasisis placed on purification or other means to protect the labile RNAmolecule. RNA is usually copied into DNA (cDNA) and then the analysisproceeds as described for DNA.

Finally, the amplification product undergoes some type of analysis,usually based on sequence or size or some combination thereof. In ananalysis by hybridization, for example, the amplified DNA is passed overa plurality of detectors made up of individual oligonucleotide detector“probes” that are anchored, for example, on electrodes. If the amplifiedDNA strands are complementary to the probes, stable bonds will be formedbetween them and the hybridized detectors can be read by observation bya wide variety of means, including optical, electrical, magnetic,mechanical or thermal means.

Other biological molecules are analyzed in a similar way, but typicallymolecule purification is substituted for amplification and detectionmethods vary according to the molecule being detected. For example, acommon diagnostic involves the detection of a specific protein bybinding to its antibody or by a specific enzymatic reaction. Lipids,carbohydrates, drugs and small molecules from biological fluids areprocessed in similar ways. However, we have simplified the discussionherein by focusing on nucleic acid analysis, in particular DNAamplification, as an example of a biological molecule that can beanalyzed using the devices of the invention.

The steps of nucleic acid analysis described above are currentlyperformed using different devices, each of which presides over oneaspect of the process. The use of separate devices increases cost anddecreases the efficiency of sample processing because transfer timebetween devices is required, larger samples are required to accommodatesample loss and instrument size, and because qualified operators arerequired to avoid contamination problems. For these reasons anintegrated microreactor would be preferred.

For performing treatment of fluids, integrated microreactors ofsemiconductor material are already known. Microchannel arrays are widelyused in different systems such as medical systems for fluidadministration, devices for biological use for manufacturingminiaturized microreactors, in electrophoresis processes, in DNA chipand other array applications, in integrated fuel cells, ink jetprinters, and the like. Microchannels are used also, for example, forthe refrigeration of devices located above microchannels.

One application of interest is the use of microchannels to make aminiaturized microreactor for diagnostic uses (see especially, U.S. Ser.No. 10/663,268 filed Sep. 16, 2003 and references cited therein, eachwhich incorporated by reference in their entirety). A number of suchdevices are described for the amplification of nucleic acid, such as DNAor RNA, or for other biological tests, such as immunological detectionof antigens in a biological sample. The microreactor can be combinedwith one or more integrated sample pretreatment chamber, micropump,heater, and also with integrated sample analysis features, such as anarray of nucleic acid or antibody detectors. Such devices are describedin more detail in U.S. Ser. No. 10/663,268, and related patents orapplications.

However, complex procedures are traditionally required in order to forma microchannel system. In particular, conventional processes for formingembedded microchannels require so-called wafer bonding or openingstructures from the backside of the wafer back.

A process for forming microchannels is described for example in the U.S.Pat. No. 6,376,291 granted on Apr. 23, 2002. In particular, thisdocument describes a process for forming in a monocrystalline siliconbody an etching-aid region for the monocrystalline silicon wherein anucleus region is provided, surrounded by a protective structure andhaving a port extending along the whole etching-aid region.

According to the '291 patent, a polycrystalline layer is grown above theport in order to form a cavity completely embedded in the resultingwafer. Although advantageous from many aspects, the process described bythe '291 patent is rather complex and it does not allow a completelycrystalline final microstructure to be obtained.

The technical problem underlying the present invention is to provide aprocess for forming microchannels, having such structural and functionalcharacteristics as to overcome the limits and drawbacks still affectingthe processes according to the prior art.

SUMMARY OF THE INVENTION

The solution underlying the present invention is to use trenchstructures to obtain deep silicon cavities. A small surface port is usedas a precursor for forming microchannels in an integrated structure.Through the port, a trench is defined and then etched to form themicrochannel structure. The port is then closed by silicon to thusobtain a completely crystalline final structure.

In accordance with an embodiment of the invention, an integratedstructure comprises at least a monocrystalline silicon substrate whereinat least one microchannel is formed which is nearly entirely buriedinside said substrate.

In accordance with an embodiment of the invention, a fluid microchannelburied in an integrated structure comprising a monocrystalline siliconsubstrate comprises an anisotropically formed trench in said substrate,and a completely monocrystalline structure which closes a top of theanisotropically formed trench to define the fluid microchannel.

In accordance with another embodiment, an integrated structure fluidmicrochannel comprises an elongated trench formed in a monocrystallinesilicon substrate and anisotropically etched to obtain a deep cavitycharacterized by an elongated surface port, and a closure of at least aportion of the elongated surface port of the elongated trench to definewith the deep cavity in the silicon substrate a tunnel-likemicrochannel.

In accordance with another embodiment, an integrated structuremicrochannel comprises a narrow elongated trench formed in amonocrystalline silicon substrate and anisotropically wet etched to forma microchannel structure having a generally rhombohedral cross-sectionalshape with a top port substrate surface opening, and a closure of thetop port substrate surface opening of the microchannel structure toenclose the microchannel structure.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the method and apparatus of the presentinvention may be acquired by reference to the following DetailedDescription when taken in conjunction with the accompanying Drawingswherein:

FIG. 1 schematically shows a section of an integrated structure with atleast a microchannel realized with the process according to theinvention;

FIGS. 2, 3A, 3B and 4 are micrographs of the integrated structure ofFIG. 1 in different steps of the process according to the invention;

FIG. 5 schematically shows an integrated structure with microchannelsrealized according to an alternative embodiment of the process accordingto the invention;

FIGS. 6A-6F schematically show an integrated structure withmicrochannels in different steps of a further alternative embodiment ofthe process according to the invention; and

FIGS. 7A and 7B show micrographs of the final integrated structure withmicrochannels realized with the process according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The invention relates particularly, but not exclusively, to a processfor realizing miniaturized microchannels buried in a completelymonocrystalline array and the following description is made withreference to this field of application for convenience of illustrationonly.

With reference to the drawings, and particularly to FIG. 1, anintegrated structure comprising a plurality of microchannels 10 formedaccording to the invention is globally and schematically indicated withreference 1.

In particular, the integrated structure 1 comprises a monocrystallinesilicon substrate 2 whereon a monocrystalline silicon layer 3 is grown.

The monocrystalline silicon layer 3 is obtained in turn by epitaxialgrowth on convenient cavities (rhombohedral in the example shown) ofsaid microchannels 10 without using coverings.

Advantageously, according to the invention, microchannels 10 arecompletely buried in the substrate 2 and the final integrated structure1 is completely monocrystalline.

The steps of the process according to the invention for forming buriedmicrochannels 10 in a completely monocrystalline integrated structure 1are now described. As it will be seen in the following description,advantageously, according to the invention, these miniaturized channelsare completely obtained through surface micromachining processes.

The process for forming buried microchannels 10 in an integratedstructure 1 according to the invention comprises the steps of:

-   -   providing a monocrystalline silicon substrate 2;    -   forming on the substrate 2 surface a silicon nitride mask (Hard        mask) through a CVD deposition technique; and    -   opening of a window having a convenient width L through        photolithographic systems and following plasma etching.

In particular, as it is schematically shown in FIG. 2, above thesubstrate 2 a window is opened having a width L of about 1 mm and adepth H of about 9 mm along the substrate 2 direction, indicated infigure with the arrow F.

Advantageously according to the invention, the process provides afollowing plasma etching which uses the hard mask to form deep trenches4 in the substrate 2, as shown in FIG. 2. Trenches 4 have side walls 4Aand 4B which are substantially orthogonal to the substrate 2 surface.

The resulting structure then undergoes a further anisotropic wetetching, for example with a TMAH or KOH solution.

It is worth noting that solutions with different KOH or TMAHconcentrations etch the monocrystalline silicon of the substrate 2 withspeeds which highly depend on the crystallographic orientations and thedopant concentration of the substrate 2 itself. It is thus possible, byusing a TMAH- or KOH-solution-etching, to form highly controllable andreproducible three-dimensional microchannels 10.

Advantageously according to the invention, trenches 4 are the precursorsof microchannels 10.

The integrated structure 1, after the anisotropic etching step, has theshape shown in FIGS. 3A and 3B, wherein a single microchannel or aplurality of microchannels are shown, respectively.

Advantageously according to the invention, the resulting microchannels10 have a rhombohedral shape.

In particular, the original shape of trenches 4 (shown in FIG. 2) turnsinto a pair of so-called rotated v-grooves V1 and V2, orthogonal to thesurface S of the substrate 2 and defining rombohedron-shapedmicrochannels 10, as shown in FIG. 3A.

In other words, a bottleneck-shaped deep cavity is obtained, which has asmall port on the surface S of the substrate 2.

In practice, while the etching time passes, because of the presence of aso-called under cut under the hard mask on the substrate 2 surface,microchannels 10 open upwardly changing the symmetry between the upperand lower part of their cavity, as schematically shown in FIG. 4.

It is, however, possible, by limiting the etching time, to obtainconveniently-sized microchannels by enlarging the depth of originaltrenches 4. In the alternative, it is possible to exploit the so-calledetch stop effect by using as hard mask a heavily doped monocrystallinelayer, as schematically shown in FIG. 5, wherein the substrate 2 andmicrochannels 10 are covered by a heavily doped hard mask layer capableof reducing under cut effects even when the substrate 2 etching timepasses.

In a preferred embodiment, the layer 5 has a dopant concentration (forexample, boron) higher than 10¹⁹ atoms/cm³.

It is also possible to use a predeposition on trench 4 walls of a layerof material 6 having a low etching speed (as, for example, the nitride).

In particular, this alternative embodiment of the process according tothe invention provides a deposition of a nitride layer 6 followed by aplasma etching effective to open a region 7 at the trench 4 base, asshown in FIGS. 6A to 6F.

The process for realizing buried microchannels 10 in an integratedstructure 1 according to this alternative embodiment of the inventioncomprises the steps of:

-   -   providing a monocrystalline silicon substrate 2;    -   growing a monocrystalline silicon layer 3 above the substrate 2;        and    -   forming a mask by means of a photoelectric film 8 above the        monocrystalline silicon layer 3, as schematically shown in FIG.        6A.

The process provides thus the steps of:

-   -   opening a plurality of windows through photolithographic systems        and following plasma etching (FIG. 6B); and    -   forming a plurality of trenches 4 in correspondence with the        plurality of windows (FIG. 6C).

Advantageously this alternative embodiment of the process according tothe invention, provides therefore a deposition step of a nitride layer 6(FIG. 6D), a removing step of the layer 6, an etching step of thesilicon substrate in a lower part 9 of trenches 4 (FIG. 6E) and a plasmaetching step effective to open a plurality of regions 7 at the trench 4base (FIG. 6F).

In particular, the plasma etching step to open regions 7 at the trench 4base is activated only in the area wherein the nitride layer 6 has beenremoved. It is essentially a so-called SCREAM process, wherein trench 4walls are protected to localize the etching only under the trench base.

Even using this alternative embodiment of the process according to theinvention, deep regions 7 are thus obtained, which have however a smallsurface opening in correspondence with the opening areas of trenches 4.

Advantageously according to the invention, trenches 4 are used for ananisotropic etching effective to obtain rhombohedral microchannels. Theshape obtained is due to the different etching speeds of the differentcrystallographic directions.

The side walls 4A and 4B of trenches 4 undergo the etching anisotropicaction and the erosion continues with different etching speeds due tothe different atom coordination (in terms of bond quantity of siliconatoms directed towards the substrate).

In particular, atoms on planes of the (100) type have coordination two(i.e., two bonds directed towards the substrate), whereas atoms onplanes of the (111) type have coordination three (i.e., three bondsdirected towards the bulk); that is that they are more bonded.

Trenches 4 are directed along the directions (110) on the wafer surfaceof the (100) type. Planes (111) find on the wafer surface just thedirection (110) and they are rotated with respect to the normal to thesurface by about 54.7°.

In particular two planes are present, which pass in the upper part oftrenches 4 and two planes passing in the lower part. All atoms alongthese directions have coordination three.

Advantageously according to the invention, the process starts by erodingthe atoms having the lowest coordination which are characterized by ahigher speed. After reaching the directions of planes (111) passingthrough/from the upper part and through/from the lower part of trenches4, speed decreases by about a hundred times since it finds only atomswith coordination three, therefore it continues with the etching speedof planes (111) as shown in FIGS. 3A and 3B. In particular, amicrochannel 10 opened towards the substrate 2 surface is obtained.

Advantageously, according to the invention, deep silicon cavities arethus obtained, being characterized by a small surface port whereto it ispossible to apply a silicon deposition step to obtain a monocrystallinestructure.

In other words, microchannels 10 have a bottle-section-shaped orrhomohedral precursor (obtained as above described) which is easilyclosed epitaxially in one embodiment or closed by deposition of a layersuch as an oxide, polysilicon, nitride or other convenient material.

Advantageously according to the invention, the process provides afurther epitaxial new growth step corresponding to the material used toclose the upper part of the microchannel 10, as shown in FIG. 7A. It isthus possible to obtain completely buried monocrystalline siliconmicrochannels 10.

FIG. 7B shows for completeness the channel profile before (10A) andafter (10B) the epitaxial new growth step. It happens thus that themonocrystalline material deposition occurs consistently also inside themicrochannel 10.

It is also possible to close the upper part of microchannels by usingother deposition techniques such as oxide or polysilicon or nitridedeposition.

In conclusion, the process for realizing microchannels 10 buried in anintegrated structure 1 according to the invention allows, thanks to theresulting etching form, the structure of the microchannel under thesubstrate 2 surface to be enlarged, but to keep, at the same time, theetching port small by means of trenches 4. The surface microchannelclosing is thus performed by growing epitaxially the material.

Advantageously, according an embodiment of the invention, the integratedstructure 1 is completely epitaxial even above microchannels 10 and itis performed by exploiting a deep cavity characterized by a smallsurface opening, which can be obtained in several kinds of processes, aswell as an easy epitaxial new growth of this cavity.

Although preferred embodiments of the method and apparatus of thepresent invention have been illustrated in the accompanying Drawings anddescribed in the foregoing Detailed Description, it will be understoodthat the invention is not limited to the embodiments disclosed, but iscapable of numerous rearrangements, modifications and substitutionswithout departing from the spirit of the invention as set forth anddefined by the following claims.

1. An integrated structure, comprising: a monocrystalline siliconsubstrate within which at least one microchannel is formed which isnearly entirely buried inside said substrate.
 2. The integratedstructure according to claim 1, wherein the microchannel has a generallyrhombohedral cross-sectional shape.
 3. The integrated structureaccording to claim 1, further comprising an epitaxially grown siliconlayer above the silicon substrate to completely enclose the microchannelin monocrystalline silicon.
 4. The integrated structure according toclaim 1, further comprising a layer above the silicon substrate to closecompletely enclose the microchannel.
 5. The integrated structureaccording to claim 4, wherein the layer is an oxide, polysilicon ornitride deposition effective to close an upper part of said microchanneland completely bury the microchannel.
 6. A fluid microchannel buried inan integrated structure comprising a monocrystalline silicon substrate,comprising: an anisotropically formed trench cavity in said substrate;and a completely monocrystalline structure which closes a top of theanisotropically formed trench cavity to define the fluid microchannel.7. The fluid microchannel of claim 6 wherein the trench cavity has agenerally rhombohedral cross-sectional shape.
 8. The fluid microchannelof claim 6 wherein the completely monocrystalline structure which closesa top of the anisotropically formed trench cavity is epitaxially grownmonocrystalline silicon.
 9. An integrated structure fluid microchannel,comprising: an elongated trench formed in a monocrystalline siliconsubstrate and anisotropically etched to obtain a deep cavity having anarrow elongated surface port along a top surface of the substrate; anda closure of at least a portion of the narrow elongated surface port ofthe elongated trench to define with the deep cavity in the siliconsubstrate a tunnel-like microchannel.
 10. The fluid microchannel ofclaim 9, wherein the closure comprises epitaxial new growth of themonocrystalline silicon substrate which closes the narrow elongatedsurface port and buries the deep cavity in monocrystalline silicon. 11.The fluid microchannel of claim 9, wherein the closure comprises anoxide, polysilicon or nitride deposition effective to close the narrowelongated surface port and bury the deep cavity.
 12. An integratedstructure microchannel, comprising: a narrow elongated trench formed ina monocrystalline silicon substrate and anisotropically wet etched toform a microchannel cavity structure having a generally rhombohedralcross-sectional shape with a top port substrate surface opening; and aclosure of the top port substrate surface opening of the microchannelstructure to enclose the microchannel cavity structure.
 13. Themicrochannel of claim 12 wherein the closure comprises epitaxially grownmonocrystalline silicon on a surface of the substrate to enclose themicrochannel structure in monocrystalline silicon.
 14. The microchannelof claim 12 further comprising a mask above the monocrystalline siliconsubstrate with an opening therein at the location of the trench.
 15. Themicrochannel of claim 14 wherein the mask is a heavily dopedmonocrystalline layer deposition.
 16. The microchannel of claim 12wherein the narrow elongated trench has a width at a surface of themonocrystalline silicon substrate of about 1 micrometer.
 17. Themicrochannel of claim 12 wherein the closure comprises a deposited layerof material taken from the group consisting of a polysilicon, a nitrideor an oxide.